The boiling water reactor (BWR) is a light water nuclear reactor used for the generation of electrical power. The BWR uses demineralized water as a coolant and neutron moderator. In a BWR, heat is produced by nuclear fission in the reactor core. The heat causes the cooling water to boil, producing steam, which is directly used to drive a steam turbine. The steam is then cooled in a condenser and converted back to liquid water. This water is then returned to the reactor core, completing the loop.
BWR internal surfaces are most commonly formed from one of the three major structural materials used in the nuclear industry. These are stainless steel (typically type 304), INCONEL® nickel-chromium-iron alloy 600 and INCONEL® alloy 182. These internal surfaces are subjected to significant tensile stresses during operation, including elevated temperatures and/or elevated pressures, and are prone to stress corrosion cracking (SCC).
The presence of noble metals deposited onto reactor internal surfaces can reduce the internal surface susceptibility to stress corrosion cracking. A simple method of applying a noble metal such as platinum (Pt) to reactor internals involves adding a solution of a noble metal compound into reactor water to cause deposition of noble metal onto contacted surfaces. This noble metal chemical addition (NMCA) technology is now widely used in BWR utilities. The NMCA treatment of surfaces has drastically lowered the hydrogen demand necessary for intergranular stress corrosion cracking (IGSCC) protection of the materials forming the internal surfaces of the reactor. A variation of this methodology for applying a noble metal on all reactor surfaces employs the reactor coolant water as the medium of transport for depositing the noble metal, a process called on-line NMCA or OLNC. The noble metal surface doping results in reactor surfaces that exhibit noble-metal-like behavior that are more resistant to reactor conditions and show reduced levels of stress corrosion cracking. These noble metal applications can be applied to most of the structural materials used in nuclear power generation.
A majority of the BWRs in the United States, and a smaller number abroad, employ NMCA in combination with low hydrogen water chemistry (HWC) that improves the effectiveness of recombination of H2 and oxidants to achieve low electrochemical corrosion potentials (ECP) (Hettiarachchi et al., CORROSION/95 Paper#95410, NACE International, Orlando, Fla., March 1995.). Typical NMCA applications (injection of noble metals into the reactor water) are performed just prior to an outage at a temperature of 240 to 290° F. (116 to 148° C.) (Hettiarachchi et al., 7th Int. Conference on Env. Degradation of Materials in Nuclear Power Systems-Water Reactors, August 7-10, Breckenridge, Colo., 1995; Hettiarachchi et al., 8th Int. Conference on Env. Degradation of Materials in Nuclear Power Systems-Water Reactors, August 10-14, Amelia Island, Fla., 1997; In-Plant Demonstration of NMCA at Duane Arnold Energy Center, BWRVIP-43, EPRI-TR108702, September, 1997; Hettiarachchi, 10th Int. Conference on Env. Degradation of Materials in Nuclear Power Systems-Water Reactors, August 5-9, Lake Tahoe, Nev., 2001). All added chemical species are cleaned-up during application and during the outage, before the plant resumes its start-up operation.
NMCA has been further developed so that noble metals at very low concentrations (parts per trillion; ppt) can be injected into the coolant while the plant is operating at full power. This process of on-line NMCA (OLNC) has been applied at a reactor water temperature of 530 to 540° F. (277 to 282° C.) (Hettiarachchi and Diaz, International Water Chemistry Conference, Seoul, Korea, October, 2006; Hettiarachchi and Diaz, 15th International Conference on Nuclear Engineering (ICONE-15), April 22-26, Nagoya, Japan, 2007; Hettiarachchi et al., 13th International Conference on Environmental Degradation of Materials in Nuclear Power Systems, Whistler, Canada, Aug. 19-23, 2007; Hettiarachchi et al., 14th International Conference on Environmental Degradation of Materials in Nuclear Power Systems, Virginia Beach, Va., August, 2009). In addition, OLNC is expected to deposit noble metal inside cracks more efficiently because of their more open nature during plant operation and the higher coolant flow rates. The advantage of NMCA or OLNC is that they require very little hydrogen addition into the feedwater (0.15 to 0.35 parts per million; ppm) to achieve low electrochemical corrosion potentials (ECPs), thus minimizing operating dose rate concerns. OLNC uses the electrocatalytic effect of platinum to efficiently recombine O2 and H2O2 with H2 on the metal surface at low feedwater hydrogen concentrations.
OLNC uses just one noble metal chemical (Na2Pt(OH)6), and the first OLNC plant demonstration was successfully completed in 2005 while the reactor was in power operation (Hettiarachchi and Diaz, International Water Chemistry Conference, Jeju Island, Korea, October, 2006). When injected into BWR environments, the noble metal particles deposit on oxidized stainless steel surfaces and lower the corrosion potential in the presence of low hydrogen, which decreases the propensity for IGSCC.
A unique feature of OLNC is the achievement of low ECP within 8 to 24 hours of the addition of parts per trillion levels of platinum into the feedwater of operating BWRs (Hettiarachchi et al., 13th International Conference on Environmental Degradation of Materials in Nuclear Power Systems, Whistler, Canada, Aug. 19-23, 2007). This response has been observed at many OLNC BWRs in both internal (reactor recirculation system or RRS) and external ECP measurement locations (material monitoring system or MMS) (Hettiarachchi et al., 14th International Conference on Environmental Degradation of Materials in Nuclear Power Systems, Virginia Beach, Va., August, 2009).
The fact that ECP reaches low values within a short time during the OLNC application indicates that the amount of platinum loading required to generate catalytic activity on stain less steel surfaces is quite small. In fact, the amount of platinum loading after a 10 day OLNC application has also been quite low (0.01 μg/cm2) in many instances posing a challenge to measuring the low platinum loadings. The actual amount of platinum loading required to achieve low ECPs in the presence of low hydrogen can in fact be a factor of ten lower (0.001 μg/cm2), based on an analysis performed to determine the correlation between the surface ECP and platinum loading (Hettiarachchi and Wehlage, BWRVIP-238, EPRI Report 1020875, January, 2010). Measuring this amount of platinum on an MMS surface by the aqua regia digestion and analysis is beyond the capability of an ICPMS by the current approach, unless the solution is concentrated by evaporation to elevate the platinum concentration in solution to measurable levels. However, ECP is able to detect the catalytic activity even at these low loading levels. Unfortunately, it is observed that ECP is not sensitive enough to detect the difference between a platinum loading of 0.001 and 0.005 μg/cm2.
FIG. 5 shows a possible explanation for why the actual platinum loading required for electrochemical corrosion potential (ECP) reduction (0.001 μg/cm2) might be much lower than the amount of platinum loading that is measured by stripping of the oxide, digestion and analysis by inductively coupled plasma mass spectrometry (0.01 to 0.05 μg/cm2). In FIG. 5, the oxidized surface 540 of a stainless steel substrate 505, as might be used in a material monitoring system, is depicted. The oxidized stainless steel surface 540 is associated with catalytically reactive platinum 500 in the catalytically accessible surface layer 530, as well as less-reactive platinum 502 contained in the catalytically inaccessible inner layer 520 not directly exposed to the reaction solution.
The less-reactive platinum 502 in the catalytically inaccessible inner layer 520 is underneath larger crud/oxide particles 510. The low ECP response is a surface property that is controlled by the catalytically active outer surface layer 530 containing catalytically accessible platinum 500, while the total platinum loading (as measured by oxide stripping, digestion and inductively coupled plasma mass spectrometry) analyzes the platinum contained in both the catalytically accessible surface layer 530 as well as platinum in the catalytically inaccessible inner layer 520 in the bulk of the crud/oxide 510 found in the catalytically inaccessible inner layer 520. Catalytically inactive (or less reactive) platinum 502 in the inner platinum layer 520 contributes little, if any, to the observed ECP response.
Thus, the parameter that reduces electrochemical corrosion potential following NMCA or OLNC is most likely the surface catalysis and deposition of platinum that occurs in the outer layer 530, not the platinum loading in the crud/oxide 510 found in the inner layer 520. It is possible that there might be instances when the measured platinum loading shows significant values, but the actual ECP response might be sluggish if most of the platinum resides in the oxide layer underneath the surface layer 530. Therefore, relying on the total platinum loading (as measured by inductively coupled plasma mass spectrometry, or ICPMS) could lead to potential errors, while a surface catalytic measurement would unequivocally correlate more closely to the low ECPs. Hettiarachchi and Wehlage, BWRVIP-238, EPRI Report 1020875, January, 2010.
EPRI documents BWRVIP-62 and BWRVIP-62A require that utilities that have performed noble metal chemical application (NMCA) or on-line noble metal chemical applications (OLNC) to prove that sufficient loading of the noble metal exist on plant internal surfaces. These BWRs face the challenging issue of determining the amount of noble metal on reactor internal surfaces for the purpose of obtaining inspection relief.
The current traditional approach to determine noble metal deposition involves allowing reactor water to flow through a materials monitoring system (MMS) that consists of a series of 0.5 to 0.75 inch internal diameter (ID) oxidized stainless steel tubing fitted together with SWAGELOK® fittings, during NMCA or OLNC applications. During or at the end of the application, MMS tubing sections are removed, cut in to three sections, and the internal diameter oxide containing the noble metal is stripped by dissolving with hot aqua regia digestion near boiling temperature to release the metal, resulting in a solution that is analyzed for the noble metal content using an inductively coupled plasma mass spectrometer (ICPMS). The noble metal concentration in the solution is then converted to a mass loading value (μg/cm2) by taking into account the surface area of the stripped section.
This traditional approach is labor and time intensive, and is plagued with drawbacks. First, the traditional methods are destructive, resulting in the removal of the oxide from the surfaces, for example, from a stainless steel surface. Second, the method requires digestion of the oxide in aqua regia, near boiling temperature, to dissolve the noble metal, such as platinum. Aqua regia, or nitro-hydrochloric acid, is a highly corrosive mixture of acids that is prone to mishandling. Third, the traditional assay method requires the use of ICPMS for the noble metal (e.g., platinum) analysis. ICPMS is an expensive instrumentation that is typically not available in BWR utilities. In addition to the expense, the use of ICPMS is problematic because the platinum loading results obtained have (e.g., 0.01 μg/cm2) approach the lower detection limits of the ICPMS instrumentation, thereby creating potential uncertainty in the loading values. This approach often requires the BWR utility to remove an MMS section periodically and ship to a vendor for processing and analysis, resulting in significant delays in obtaining the loading data.
What is needed in the art are rapid, reliable, non-destructive methods for assessing and quantitating the loading of noble metals such as platinum on plant internal surfaces that have performed NMCA/OLNC applications. Ideally, these methods can be conducted at the plant site. This will greatly facilitate the process for obtaining inspection relief, and would be of great value to the BWR utilities.
The present invention, in its many embodiments, provides solutions to these problems, have a number of advantages over the state of the art and provide many benefits previously unrealized in other types of methods. In addition, still further benefits flow from the invention described herein, as will be apparent upon reading the present disclosure.
Aspects of the general state of the art can be found in various sources, for example, Lin and Smith, “Decomposition of hydrogen peroxide at elevated temperatures”, EPRI Project NP-6733 (March 1990); Macinnes, “The mechanism of the catalysis of hydrogen peroxide by colloidal Pt,” J. of American Chemical Society, 36(5):878-881 (1914); Rooth and Ullberg, “Hydrogen Peroxide in BWRs”, Water Chemistry of Nuclear Reactor Systems 5, Vol. 2, British Nuclear Energy Society, London, UK (Oct. 23-27, 1989); U.S. Pat. No. 2,721,788, to Chad, entitled “A reaction for hydrogen peroxide decomposition”; U.S. Pat. No. 3,347,630, to Baumgart, entitled “Hydrogen peroxide decomposition”; and U.S. Pat. No. 5,711,146, to Armstrong and Toombs, entitled “Hydrogen peroxide decomposition.” The present invention provides advantages previously unrealized over the state of the art.